But even a balanced diet can let you put weight if you eat too much.
We know genetics disorder like Prader-Willis can cause insatiable appetite. Even with a "balanced diet" those affected by it will end-up obese.
Now the current research on GLP-1 hint at many people having a harder time feeling satiated, not to the Prader-Willis point but still worse than what is considered normal. For people who don't have the problem it is easy to think they just have more discipline / willpower and that's why they're thin. I'd like them to imagine what would happen if after eating 2 or 3 pizza slices they'd still feel like their stomach is empty. What if it's not a one time occurrence but all day every day from the moment they develop a conscience to their death bed. A little like drug addiction but you need some drug to live so it is legal and you can buy it everywhere. Still think you'd have the discipline to not go for the whole pizza?